The following invention relates to a method and an apparatus for passivating defects in semiconductor substrates, in particular silicon-based semiconductor substrates for photovoltaic elements.
Photovoltaic elements serve to convert light directly into electrical energy. For this purpose, on a p-type or n-type semiconductor an area with a respective opposite type doping is formed to form a p-n junction. When illuminated with light, charge carrier pairs are generated, which are spatially separated by a potential gradient formed by the p-n junction. The separate charge carriers can then be passed through the semiconductor and fed to an external circuit.
Photovoltaic elements, also referred to as solar cells, often use crystalline silicon as the semiconductor, wherein it is distinguishing between poly- or multi-crystalline silicon (poly-Si) and mono-crystalline silicon (mono-Si). Although higher efficiencies can typically be achieved with mono-Si, the production process of mono-Si over poly-Si is more complex and energy-intensive. In the frequently used Czochralski method, boron doping is also commonly used to produce a p-type semiconductor. Inevitably, oxygen atoms are also incorporated into the silicon crystal during production.
However, the boron atoms in combination with the oxygen atoms tend to form electrical defects also known as traps, which can adversely affect the electrical properties of the solar cells. In particular, a substantial reduction in the efficiency of a solar cell was found at high boron and/or oxygen concentrations. This is especially the case when the solar cells are illuminated for a long time, as this activates the B—O-complexes as recombination centers. One therefore also calls this effect “Light-Induced Degradation”. However, also other defects, such as defects in the crystal lattice, and metallic impurities, such as occur more frequently in poly-Si, can significantly reduce the efficiency of a solar cell under working conditions.
The problem of light-induced degradation in mono-crystalline CZ-Si is known in the art and has already been explained in detail in German patent application DE 10 2006 01920 A1. In this application, a method for stabilizing the efficiency of a Si solar cell with boron doping was proposed, in which the substrate was illuminated with light of a wavelength of less 1180 nm while the substrate was at a temperature of 50-230° C. The illumination leads to the generation of excess charge carriers and in particular also to a change in the electrical state of hydrogen H contained in the silicon crystal. In particular, the concentration of the neutral (H0) species of the hydrogen atoms is increased, which enables a passivation of uncharged defects in the crystal structure, such as boron-oxygen defects. A similar process would also take place in multi-crystalline Si, leading to the passivation of defects.
The above process has led to a stabilization of the efficiency of silicon solar cells, but is very time-consuming to achieve a long-term stabilization. The inventors have found that much higher radiation intensities than proposed in DE 10 2006 01920 A1 are advantageous for the generation of charge carriers (and a complete formation of the defects which are then to be passivated), but also found that the higher radiation intensities can also lead to temperature problems even though higher temperatures than those specified in DE 10 2008 01920 A1 are also being taken into consideration.
Starting from the known method, it is therefore an object the present invention to provide an improved method and apparatus for the passivation of defects in silicon, such as the regeneration of semiconductor substrates.